1. Field of the Invention
The present invention relates to an ultrasonic probe to be used for extra-cavity scanning or intra-cavity scanning of an object to be inspected. Further, the present invention relates to an ultrasonic diagnostic apparatus including such an ultrasonic probe and an ultrasonic diagnostic apparatus main body, and an ultrasonic endoscopic apparatus including an ultrasonic endoscope having the ultrasonic probe and an ultrasonic endoscopic apparatus main body.
2. Description of a Related Art
In medical fields, various imaging technologies have been developed in order to observe the interior of an object to be inspected and make diagnoses. Especially, ultrasonic imaging for acquiring interior information of the object by transmitting and receiving ultrasonic waves enables image observation in real time and provides no exposure to radiation unlike other medical image technologies such as X-ray photography or RI (radio isotope) scintillation camera. Accordingly, ultrasonic imaging is utilized as an imaging technology at a high level of safety in a wide range of departments including not only the fetal diagnosis in the obstetrics, but gynecology, circulatory system, digestive system, and so on.
The ultrasonic imaging is an image generation technology utilizing the nature of ultrasonic waves that the ultrasonic waves are reflected at a boundary between regions with different acoustic impedances (e.g., a boundary between structures). Typically, an ultrasonic diagnostic apparatus (or referred to as an ultrasonic imaging apparatus or an ultrasonic observation apparatus) is provided with an ultrasonic probe to be used in contact with the object or ultrasonic probe to be used by being inserted into a body cavity of the object. Alternatively, an ultrasonic endoscope in combination of an endoscope for optically observing the interior of the object and an ultrasonic probe for intracavity is also used.
In the ultrasonic probe, generally, a piezoelectric vibrator having electrodes formed on both ends of a piezoelectric material is used as an ultrasonic transducer for transmitting and receiving ultrasonic waves. When a voltage is applied to the electrodes of the vibrator, the piezoelectric material expands and contracts to generate ultrasonic waves. Further, plural vibrators are one-dimensionally or two-dimensionally arranged and the vibrators are driven by drive signals provided with predetermined delays, and thereby, an ultrasonic beam can be formed toward a desired direction. On the other hand, the vibrator receives the propagating ultrasonic waves, expands and contracts, and generates an electric signal. The electric signal is used as a reception signal of ultrasonic waves.
As a guideline of a piezoelectric material to be used for a conventional piezoelectric vibrator, one having a large vibration amplitude (large piezoelectric strain constant d33) and generating a small amount of heat at transmission of ultrasonic waves has been desired. Further, for reducing heat generation, it has been considered important that the mechanical quality factor Qm is large and the dielectric loss tan δe is small (see the following documents).
As a related technology, Japanese Patent Application Publication JP-A-5-51222 discloses a piezoelectric porcelain material that provides large vibration and generates a small amount of heat in use as an ultrasonic vibrator. In the piezoelectric porcelain material, all of the mechanical quality factor, the piezoelectric constant, and the dielectric loss are above certain levels.
Further, Japanese Patent Application Publication JP-A-6-14398 discloses an ultrasonic probe for suppressing an amount of heat generated when driven and preventing the characteristic degradation thereby. According to JP-A-6-14398, in an ultrasonic probe in which a piezoelectric element as a source of ultrasonic waves is fixed onto a backing material, an acoustic matching layer is provided on the front face of the piezoelectric element, and they are enclosed in a case, the dielectric loss of the piezoelectric element is 1.6% or less and the amount of heat generated at driving of the piezoelectric element is lowered.
Furthermore, Michael J. Zipparo, “Multilayer Ultrasound Imaging Arrays showing Improved Thermal Performance”, Proc. 2005 IEEE Ultrasonics Symposium, pp. 2235-2238 describes multilayer ultrasound imaging arrays designed to have improved acoustic power delivery capability without sacrificing imaging performance. In the document, Table 1 shows physical properties of PZT materials used for single layer ultrasound imaging arrays, and Table 2 shows physical properties of PZT materials used for multilayer ultrasound imaging arrays.